Coating removal

ABSTRACT

An indium tin oxide (ITO) coating is selectively removed from a substrate by printing a mask on the coated surface of the substrate to cover those regions of the surface on which the ITO is to remain. The substrate is then abraded to remove the ITO from the unmasked regions of the surface and the mask is removed with a solvent. The method provides quick and accurate removal of ITO without the use of acids or lasers.

BACKGROUND OF THE INVENTION

The present invention relates to a method of removing a surface coating,in particular to a method of removing an indium tin oxide coating from asubstrate for an electroluminescent display.

Electroluminescence is the emission of light by a material whensubjected to an electric field.

A typical thick film phosphor electroluminescent device comprises alayer of electroluminescent material in a dielectric matrix, sandwichedbetween two planar conducting electrodes. The electroluminescentmaterial comprises phosphor particles, typically a zinc sulphide (ZnS)powder doped with manganese (Mn), microencapsulated in a dielectricmaterial. Typically, silver- or graphite-loaded screen-printable inks,and indium tin oxide (ITO), a transparent conductive material,respectively are used to form the electrodes on a substrate such as apolyester film. When an AC voltage is applied between the electrodes,the electroluminescent material emits light.

The inventors have recently developed thick film electroluminescentdisplays in which a plurality of shaped independent electrodes areprovided on at least one side of a layer of electroluminescent material.A voltage may be applied selectively to each of these independentelectrodes to illuminate a respective region of the display. A thickfilm electroluminescent display is created by selecting theconfiguration of the independent electrodes to represent information,for example in the form of a seven-segment display or the like.

A problem associated with the manufacture of thick filmelectroluminescent displays is that the independent electrodes must beconnected electrically to a voltage source for the display. In aconvenient manufacturing technique, electrical connections are appliedas conductive tracks on the rear surface of the device, for example byscreen printing conductive ink. However, when a voltage is applied tothe conductive tracks, they can act as electrodes causing theelectroluminescent material to illuminate. Such illumination isundesirable as it detracts from the clarity of the display.

One way to prevent unwanted illumination due to the conductive tracks isto remove the ITO from the substrate in the regions opposite theconductive tracks. The ITO should be removed completely and in registerwith the other layers of the display.

Commercially available materials suitable for use as substrates inelectroluminescent displays can have very resilient coatings of ITO. Themanufacturers of such materials recommend the use of an acid for etchingaway the unwanted areas of the ITO coating. According to the recommendedprocess, the coated substrate is initially provided with an etchingmask, which is typically alkali soluble, and the substrate is immersedin acid, for up to 20 minutes, depending on the grade of ITO. Thesubstrate is then rinsed and the mask is removed with an alkalisolution. The process is messy, and difficult to control.

An alternative method of removal is laser ablation of the ITO layer.However, this method is slow because the laser is scanned across thesurface of the substrate, and the process can take a very long timewhere ITO is to be removed from a relatively large area.

SUMMARY OF THE INVENTION

The present invention provides a method for selectively removing asurface coating, in particular an indium tin oxide (ITO) coating, from asubstrate, the method comprising:

providing a substrate having a coated surface;

applying a mask to the coated surface to cover those regions of thesurface on which the coating is to remain; and

abrading the substrate to remove the coating from the unmasked regionsof the surface.

The inventor has found that abrasion of the masked substrated provides aquick, accurate and relatively simple method of removing a coating suchas ITO from a substrate, in which it is not necessary to use hazardouschemicals such as acids or expensive and slow laser equipment.

The substrate may be any suitable material, for example a plastics film,in particular a transparent plastics film. In the preferred embodiment,the substrate is a polyester or polyethylene teraphthalate (PET) film.

The coating may be any suitable coating. In the preferred embodiment,the coating is indium tin oxide (IYO) but other transparent conductivecoatins, such as gold and silver, may also bus used in the context ofthe invention.

Advantageously, the mask is printed, for example screen printed, ontothe substrate. In the production of electroluminescent displays, thelayers of conductive, insulating and electroluminescent material may bescreen printed onto the substrate. Thus, the mask may be printed in thesame way, so that registration of the mask can be assured in the sameway as for the other layers.

After the coating has been abraded, the mask may be removed, for examplewith a solvent. However, it is not necessary for the mask to be removed.For example, it would be possible for the mask to be constituted by oneor more of the layers of the electroluminescent display. In this case,abrasion would take place after the relevant layer had been applied tothe substrate.

The coating may be abraded in any suitable manner, for example usingrotary brushes or the like. In the case of a flexible substrate, thesubstrate may be held during abrasion by suitable means such as a rigidvacuum bed a vacuum conveyer. Rotary or oscillating brushes may be used,in combination with abrasive media, such as pumice, silicas, aluminiumoxide, glass powder, diamond slurry, etc.

Abrasive pads may be used for abrading the coating. In particular, padsof Scotchbrite (trade mark) material have been found to be particularlyeffective. In one trial, a flap roller (a cylinder made up of a largenumber of sheets of abrasive material) was found to be very successful.The abrassive pads and/or brushes can be used wet or dry. A furtheralternative for abrading the coating is a sanding belt, flat disc orsimilar device.

The abrasion may be carried out in more than one step or by more thanone method. For example, a sequence of the above described abrasionmethods may be used, possibly with a fine abrasion process following acoarse abrasion process. The abrasion devices may operate at differentangles, for example two rollers at 45 degrees along the length of themachine.

In the case of an electrically conductive coating, it is not necessaryfor the coating to be completely removed, provided that it issufficiently disrupted in the unmasked regions to prevent its effectiveoperation as a conductor. Thus, the abrasion step may be chosen to scoreor grain the conductive coating to break up the conductive pathwaysthrough the conductor. In one particular arrangement, the graining maybe carried out in two directions to form isolated squares or diamonds ofthe coating on the substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A specific, non-limiting example of a process according to an embodimentof the invention will now be described for the purposes of illustrationonly.

EXAMPLE

A sample of transparent polyethylene teraphthalate (PET) film coatedwith indium tin oxide (ITO) produced by CP Films, Inc. of Martinsville,Va., USA as Courtaulds OC 80 film (sheet resistance 80 ohms square) wasabraded in accordance with the invention. A mask of XV 1000 UV-curingetch resist blue, available from Coates Electrographics of Bath, UK, wasscreen printed onto the film and cured.

The masked film was passed twice under a rotating flap roller ofScotchbrite CPFB AMED material available from 3M of St. Paul, Minn.,USA, at a brush speed of 14 m/s, a feed speed of 2 m/s and a lateraloscillation rate of 200 cycles per minute.

The ITO coating was completely removed in the unmasked areas withoutdamaging the etch resist. The surface of the PET was lightly grained bythe abrasion, but this is not a disadvantage of the process, as theareas in which the ITO is removed are generally masked by a printoverlay in the electroluminescent device.

The etch resist was removed by wiping with a dilute sodium hydroxidesolution.

The processing of the film according to this example is relatively cheapand quick, because the processing speed is unaffected by the amount ofITO that must be removed.

As an alternative to the flap roller, a large diameter Scotchbrite padcould be used to abrade the ITO coating. Timesavers International BV ofGoes, The Netherlands make a machine under the designationGrindingmaster that is able to use a 900 mm diameter pad.

In summary, an indium tin oxide (ITO) coating is selectively removedfrom a substrate by printing a mask on the coated surface of thesubstrate to cover those regions of the surface on which the ITO is toremain. The substrate is then abraded to remove the ITO from theunmasked regions of the surface and the mask is removed with a solvent.The method provides quick and accurate removal of ITO without the use ofacids or lasers.

1. A method of producing an electroluminescent display in the form of anelectroluminescent layer sandwiched between a substrate and a pluralityof independent electrodes and conductive tracks for activating theelectroluminescent layer, the method comprising the steps of: providinga transparent conductive coating over a surface of the substrate;applying a mask to the coated substrate surface to cover regions of thesurface on which the conductive coating is to remain and to leave barethose regions of the coating which are in register with the conductivetracks; and removing by abrasion the coating where the mask is notpresent.
 2. A method as claimed in claim 1, wherein the abrasion isconducted by mechanical rubbing means.
 3. A method as claimed in claim2, wherein the abrasion is conducted with abrasive pads.
 4. A method asclaimed in claim 3, wherein the abrasive pads form a flap roller.
 5. Amethod as claimed in claim 2, wherein the abrasion is conducted with aflat disc.
 6. A method as claimed in claim 1, further comprisingsufficiently removing the coating of the unmasked region to the extentnecessary to prevent the coating from operating as a conductor.
 7. Amethod as claimed in claim 1, further comprising completely removing thecoating from the unmasked region.
 8. A method as claimed in claim 1,further comprising printing the mask onto the substrate.
 9. A method asclaimed in claim 1, further comprising the step of removing the mask.10. A method as claimed in claim 9, further comprising removing the maskwith a solvent.